The Mach External Pager allows clients to replace the paging mechanism. More recent work, McNamee and Krueger allow client replacement of the paging policy as well; Scheduler Activations share the job of scheduling between clients and the system; Apertos allows these and other aspects of operating system implementation to be client-controlled. Object-Oriented operating systems under development provide these and other kinds of control as well.
What is going on? Why are all these people abandoning the traditional notion of hiding implementation? Have they lost their senses? Or is there some deeper underlying commonality that explains this trend? Why are object-oriented techniques so tied up with this? Why do some people use terms like ``meta'' and ``reflective'' when they talk about these systems?
We have developed a simple analysis that provides perspective on the situation. We will argue that there are basic dilemmas presented by the abstractions in operating systems and other software systems and that object oriented techniques provide tools to address those dilemmas.
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